I'll take Pierce based in part on the following fun fact:
In the 3-point era, 460 different players have averaged 22+ points per game in a season.
If you rank them all by WS48 Kobe's 2014-15 campaign ranks dead last.
http://bkref.com/tiny/MPfqhHe is similarly awful on other summary metrics like VORP. As awful as the Lakers were last year, they were actually far worse (by 5 points per 100 possessions) with Kobe on the court than with him off the court.
Basically if he plays a full season Kobe is on track to have one of the the worst high-volume, low-efficiency, poor-defending seasons in NBA history.
Pierce will be the exact opposite - a savvy vet who takes what the game gives him and contributes in all phases of the game. It's really no contest IMO.